The objective of this contract is to carry out a prospective cohort study of diet in relation to several major cancers, especially breast and prostate malignancies. The cohort will consist primarily of male and female members of the American Association of Retired Persons (AARP). In order to guarantee adequate heterogeneity of dietary intake for key nutrients and foods, the cohort will be established in a two-phase process. Food frequency questionnaires will initially be sent to 3.5 million AARP members. All respondents falling within the 'extreme' categories of nutrient (e.g., dietary fat) intake will then be selected for the final cohort, along with a random sample of those falling within the intermediate categories of intake. The final cohort will consist of 175,000 men and 175,000 women. Oversampling of minority AARP members will be done to ensure an adequate representation of minorities in the final cohort. Validation studies of the food frequency questionnaire will be carried out in the early months of the contract period. Follow-up will be largely passive through established state cancer registries; the initial questionnaire mailing will be to AARP members in those states selected on the basis of having registries with minimum coverage and quality. Active follow-up with record retrieval will be conducted for the small percentage of cohort members who have moved out of the cancer registry areas. The duration of follow-up is five years.